bug#18420: 24.3; interaction with external process hangs emacs

[Stephen Leake <stephen_leake@stephe-leake.org>]

More info from the debugger on the hung emacs. There are 6 threads active:

thread 6 : gdb interface
thread 5 : child process reader_thread
    used by select emulation to check for child process output
    can't see contents of 'cp' object; optimized out
        the only child process of this emacs instance is the external
        parser; ada_mode_wisi_parse

    hung at "Wait until our input is acknowledged before reading again"
        waiting for 'select()' to ack?

thread 4 : windows message handler
thread 3 : itimer thread
thread 2 : waiting on some startup?
thread 1 : main, runing lisp code, process_send_string, emacs_full_write, sys_write _write
    sys_write can only be interrupted by interrupts/signals
    emacs_full_write checks for that, handles them
    should allow for read write, unless it there's a race condition

So there is a deadlock:

thread 5 is waiting for thread 1 to ack
thread 1 is waiting for child process to read
child process is waiting for thread 5 to read
                
We need to understand how we get here.

-- 
-- Stephe